The present disclosure relates generally to organic electroluminescence (EL) display devices, and more particularly to the detection of moisture within the sealed pixel elements and sealed display panels of the EL electro-optical display devices and the like.
Electroluminescence (EL) display devices comprise of a plurality of light emitting elements (pixels) which utilize electric field light emission of solid fluorescent substance or phenomenon called electroluminescence. The luminous material layers of an EL device is commonly applied in the backlight of liquid crystal, flat panel, electro-optical displays which may be either transmissive, reflective and/or transflective. Advanced technology EL pixels utilize organic light emission diodes (OLEDs) featuring the use of organic polymer material compound layers as the luminous material layers of the devices. The use of the organic polymer layers, as opposed to the previous usage of inorganic material layers, offer improvements to the display devices' display performance, operational efficiencies, package sizing/portability, as well as reduction in power and voltage requirements.
Organic EL elements such as OLEDs are much more sensitive to degradation issues related to ambient environment conditions such as water moisture and free oxygen than inorganic EL elements. Specifically, water moisture and oxygen may cause undesired crystallization and formation of organic solids, undesired electrochemical reactions at the electrode-organic layer interfaces, corrosion of metals and the undesired migration of ionic species. It has been noted in research, that degradation due to water moisture is at least a thousand times more destructive than from free oxygen. These degradation mechanisms often manifest as the growth of dark spot defects upon the emissive display elements. Such defects may lead to performance loss, operational instability, poor color/emission accuracies, as well as shortened operational life. The dark spot defects are typically not immediately formed upon the display elements as their growth in quantity, size and location are based upon time exposure in the offending environment.
To minimize such degradation mechanisms, the organic EL elements are typically encapsulated in an attempt to prevent moisture migration to the active EL display elements. Typical encapsulation methods utilize a transparent, translucent shield covering the entire display element with an adhesive sealant used to seal the shield to the device substrate layer. For LCD as well as LED display panel devices, depending on the technology, the shield may encapsulate a plurality of display elements as opposed to, or in addition to the encapsulation of single or smaller group of display elements. It is noted that conventional sealing, encapsulation methods often trap some lower level of residual water moisture within the display device as efforts for total water moisture removal during the sealing, encapsulation processes are very difficult to accomplish.
Dark spot defects may still grow within encapsulated display elements due to various reasons. Encapsulation seals may be initially poor or themselves degrade in time, enough to lose their seal or encapsulation integrity. Poor initial seal and loss of seal integrity will allow water moisture to migrate to the organic EL display elements. The trapped, residual water moisture held within the encapsulated organic EL elements may also cause dark spot growth.
EL display device fabrication facilities typically implement environmental stress testing upon completed, fabricated display devices in attempt to accelerate the dark spots growth. Such accelerated testing methods using stress environments such as high temperatures (60-85 degrees Celsius) and high relative humidity (85-90% RH) allows the production facilities to visually inspect and judge their completed display device products for defectively encapsulated devices as well as providing relative judgment and extrapolation of seal integrity and expected lifetime. Limitation and inaccuracies to such described stress testing methods are very dependant upon and due to the manual, visual inspection procedures used to attempt the quantitative and qualitative analysis of the dark spot defects.
FIG. 1 is a top view of a typical organic EL device 100 to illustrate the application of the shield and adhesive sealant to encapsulate the display device. The device substrate 102 is shown covered with the encapsulation shield 104. The shield 104 may be a color filter or clear/transparent substrate, and covers the entire organic EL device with continuous lines of adhesive sealant 106 located on the device substrate 102 along the same directions as the length and width perimeters of the EL display device 100. The shield 104 is attached directly to the device substrate 102 utilizing the adhesive sealant 106 to encapsulate the EL display device 100. In summary, the encapsulation seal is accomplished primarily using a sealant between the bottom surface of the shield 104 and top surface of the device substrate 102. FIG. 1 also illustrates examples of dark spot defects 108 that have grown onto the display element of the EL device 100. There are two dark spots 108 shown to illustrate the randomness of quantity, size and location within the sealed display device.
FIG. 2 illustrates the top view of an organic EL display panel 200 that contains multiple EL display devices or display elements. Such a display panel contains multiple OLED elements 202 encapsulated between the shield 204 and device substrate 206. The shield 204 is sealed to the device substrate 206 with continuous lines of adhesive sealant 208 located on the device substrate 206. Dark spots 210 are shown located within the encapsulated display panel 200.
What is desirable is an improved method and/or test device for the detection of water moisture within encapsulated organic EL display devices and display panels.